Copper (II) tetraphenylporphine (CuTPP) has been found to partition favorably into dimyristoylphosphatidylcholine (DMPC) vesicles. EPR spectra of oriented DMPC membranes together with computer simulations show that the CuTPP is preferentially oriented with the plane of the molecule perpendicular to the bilayer surface at low (less than or equal to 1/100) CuTPP/DMPC molar ratio. However, for a high (ca. 1/25) CuTPP/DMPC molar ratio, the plane of the molecule becomes parallel to the membrane surface. Measurements of the collision rate between CuTPP and the nitroxide moiety of stearic acid spin labels located at different depths in the membrane using saturation recovery EPR spectroscopy led to the conclusion that the change of the CuTPP orientation is the result of self-association (crystallization) of CuTPP molecules near the lipid bilayer center. The self-association process depends both on the CuTPP/DMPC molar ratio and the time after sample preparation.[unreadable]